This invention relates to spring biased electrical contact probes in general and specifically to such probes that are of extremely small size and used in densely packed electrical test sockets.
Test sockets are used in the electrical circuit test industry as an interface between a test circuit board and the device under test, commonly called a DUT. Test sockets are normally used for a DUT that has relatively densely packed contact points. Such a device could be an integrated circuit chip, commonly called an IC. The test socket is configured to receive a particular configuration of IC. Electrical test spring contact probes in the well of the test socket extend to the other side of the socket and contact a circuit board on which the test socket is mounted. The test sockets and their loaded probes establish an electrical contact path between the DUT and the circuit board.
Particularly with complex and expensive IC""s, reliability of the test readings is of critical importance, for such DUT""s are normally costly and failing a DUT because of test socket and probe failure can lead to a needlessly high rejection rate which increases the cost of manufacture. It is normally preferred to test the IC before soldering the IC to a circuit board and then functionally testing the finished article. If the IC then fails, scrapping the finished article is extremely expensive both in terms of material and time costs. An integrated circuit of complexity with densely packed pad contacts is expensive, making accuracy and reliability of the test equipment critical.
The test equipment is even more expensive and a test socket densely packed with test probes more expensive yet. Because of the costs involved, these devices must be reliable and have long life. Test sockets produced by the assignee of the present invention are expected to provide reliable readings for over 500,000 cycles.
Probes used in the test sockets provide the interconnection between the test circuit board and the IC. These must stand up for over 500,000 cycles and are of extremely small dimension. For example, contact probes in a test socket must be approximately 0.02 inch in diameter and may have a fully extended length of 0.2145 inches. Such probes are extremely difficult to manufacture, as they typically consist of a barrel, a coiled spring within the barrel and extensible plunger extending from the barrel and biased to an extended position by the coil spring. Heretofore, the barrel has been crimped about a reduced area of the plunger, allowing the plunger to reciprocate relative to the barrel. Crimping requires an extra step of manufacture and is difficult in an item so small. Moreover, it results in reduced surface area of contact between the plunger and barrel, leading to decreased surface area of contact and greater resistance. Electrical resistance is inversely proportional to the square of the cross-sectional area of contact. By increasing the surface area of contact, there is less resistance. The inventor has determined that minimal, constant resistance in the contact probe is of great importance in increasing reliability of the testing sequence.
The present invention provides a combination probe and test socket assembly in which a probe of extremely small size, such as 0.02 inch diameter, is formed with a barrel, a hollow plunger shaft and an interior coil spring which urges the plunger into an extended position relative to the barrel. The probe is double ended; that is, the barrel has a contact tip and the plunger has a contact tip, allowing the probe to connect linearly spaced electrical test sites. The probe is designed for maximum electrical contact, leading to extremely low resistance and does not have a crimp or other conventional connecting means. The bore through the test socket for the probe is designed with upper and lower inner shoulders and the socket is of a two-piece, upper and lower section design. To fit the probe into the bore, the upper and lower sections are separated and the probe inserted therein. The upper and lower shoulders within the bore keep the probe together. The probe provides extremely low and consistent electrical resistance.
The objects of the present invention are:
to provide a probe and test socket assembly wherein the probe is of extremely high consistent and low electrical resistance;
to provide a test socket wherein the probes are of extraordinarily small size and can be densely packed for testing integrated circuit chips;
to provide such a probe which can be manufactured with a high degree of accuracy and low cost; and
to provide such a probe and test socket assembly which is able to withstand a high amount of cycles, such as 500,00 cycles before changeout.
Other objects and advantages of the invention will become apparent from the following description.